Don't know if this really works or not, but it seems to fit what you are asking for. It's hardwired, so it can't be disabled easily (unless they turn the breaker off), it can be fitted with a tamper proof cage, and it can play custom messages "YOU HAVE BEEN CAUGHT SMOKING, OUR STAFF HAVE BEEN NOTIFIED, YOU HAVE 2 HOURS TO PACK YOUR BAGS!!!".
http://www.smokersensor.com/
A nicotine test from the wall has it's problems too. A tenant intent on cheating would just question the method and application on it's use. They would either claim whatever you detected on the wall was there before they moved in, something left by the last tenant. Unless you completely repainted the walls and ceilings, and replaced all carpets and draperies. they will just say it's not from them unless you can show an identical test result claiming that spot tested negative when they moved in.
Or, they will just say they did smoke, outside the building, at work, or in their vehicles. They didn't violate the non smoking clause, but the stuff clings to their clothing and hair. So they may have leaned on the walls, sat on the floor and transferred things when they did so. Is there any rules against leaning on a wall?
I see someone intentionally cheating and denying he/she smoked a difficult thing to pin down. May be more likely to also ask neighbors who complained to help with the enforcement.